The present invention relates to an image forming apparatus such as an electrophotographic copier or a laser beam printer, and a process cartridge for use therein.
Electrophotographic image forming apparatus using an electrophotographic image forming process conventionally employ a process-cartridge method of integrating an electrophotographic photosensitive member with a process means acting thereon to form a cartridge that can be installed in and removed from an image forming apparatus. This process-cartridge method enables a user to perform the maintenance of the apparatus without relying on service personnel, thereby drastically improving operability. Thus, this process-cartridge method is widely used for electrophotographic image forming apparatus.
The process-cartridge method comprises integrating a charging or cleaning means with a developing means and an electrophotographic photosensitive drum to form a cartridge that can be installed in and removed from the image forming apparatus main body. Alternatively, at least one of the charging and cleaning means is integrated with the developing means or the electrophotographic photosensitive drum to form a cartridge that can be installed in and removed from the image forming apparatus main body. The process-cartridge method may alternatively comprise integrating at least the developing means and the electrophotographic photosensitive member together to form a cartridge that can be installed in and removed from the image forming apparatus main body.
Such a process-cartridge comprises a developing member and a developer containing toner, functioning as a developing means.
FIG. 8 shows a conventional example of a laser printer as an image forming apparatus to which the process-cartridge method is applied. This image forming apparatus comprises a photosensitive drum 1 functioning as an electrophotographic photosensitive member, an exposure device 2 functioning as a static-latent-image forming means, a developing device 3 functioning as a developing means, a transfer member 4 functioning as a transfer means, a cleaning device 5 functioning as a cleaning means, a charging member 6 functioning as a charging means, a fixing device 7, a sheet feeding cassette B in which transfer materials to be supplied are placed, and a sheet feeding device 8. In FIG. 8, reference character P denotes a passage through which transfer materials are conveyed, and reference character L denotes a laser beam from the exposure device 2. In this case, the photosensitive drum 1, the developing device 3, the cleaning device 5, and the charging member 6 are integrally supported to form a process cartridge.
The exposure device 2 turns on and off a laser beam L corresponding to image information to apply it to a surface of the photosensitive drum 1, which has been charged to a desired potential by the charging member 6. Thus, the charges are eliminated to form a static latent image on the photosensitive drum 1.
The developing device 3 comprises a cylindrical metal developer holding member (hereinafter referred to as a xe2x80x9cdeveloping sleevexe2x80x9d) 31 arranged opposite to the photosensitive drum 1 in a developing container. The developing sleeve 31 is coated with coarse particles such as polymethyl methacrylate resin (PMMA) or spherical carbon particles and a thin conductive layer composed of a composite material consisting of a binding resin, carbon black, and carbon graphite. An elastic blade 32 having an elastic member, such as urethane rubber, is arranged as a developer regulating member to form a nip portion between the developing sleeve 31, and the elastic blade 32 (hereinafter referred to as a xe2x80x9cdeveloping bladexe2x80x9d), so that the nip portion is used to form a thin layer of a developer on the developing sleeve 31, thereby allowing the developer to be charged. The toner in the developer is supplied from the developing sleeve 31 depending on the static latent image to form a toner image on the photosensitive drum 1.
In general, the developer is produced using as materials a binding resin that fixes the developer to a transferred material, various coloring materials that provide the tones of toner, and a charge-control agent that applies charges to particles. In the case of a one-component developer, such as those shown in Japanese Patent Application Laid-Open Nos. 54-42141 and 55-18656, the toner itself comprises a magnetic material so as to be conveyable. Furthermore, another additive such as a releasing agent, is added to and dry-mixed with the toner as required. Subsequently, the mixture is melted and kneaded by a general-purpose kneading apparatus, such as a roll mill or an extruder, and is then cooled and solidified. Then, the kneaded mixture is crushed by any crushing apparatus, such as a jet stream crusher and a mechanical collision crusher, and the fine crushed pieces so obtained are introduced into any pneumatic classifier for classification. Thus, toner particles with an equal required size are obtained, and a fluidizing agent or a lubricant is dry-mixed with the particles to obtain toner for use in image formation.
Further, for a two-component developer, any magnetic holding member and the above-described toner are mixed together, and the mixture is used to form an image.
The transfer material 4 allows a toner image on the photosensitive drum 1 to be transferred to the surface of the transfer material. This unfixed toner image on the transfer material is heated and pressurized by the fixing device 7 so as to be permanently fixed to the transfer material, and the transfer material is then discharged from the image forming apparatus.
On the other hand, toner or paper dust remaining on the photosensitive drum 1 after transfer is cleaned by the cleaning device 5. Further, a residue checking bar 11 is used to detect a change in the static capacity between the bar and the developing sleeve 31 to detect the amount of remaining toner.
A developing section formed of the photosensitive drum and the developing sleeve, which are opposite to each other, depends on the construction of the developing device. Accordingly, the same developing device construction may not ensure a sufficient developing capability for an image forming apparatus with an increased speed (process speed). FIG. 9 shows the relationship between the number of sheets printed and the sheet-image-reflection density as observed if conventional toner, having a lower circularity as described above, is used as a developer. Here, a reflection densitometer X-Rite504 manufactured by X-Rite Co., Ltd. was used to measure the image-reflection density. In this plot, the squares denote the transition of the density observed at a process speed (the peripheral speed of the photosensitive member) of 100 mm/sec., the triangles denote the transition of the density observed at a process speed of 150 mm/sec., and the circles denote the transition of the density observed at a process speed of 200 mm/sec. The construction of the developing device is as shown in the conventional example; the photosensitive drum had a diameter of 30 mm, the developing sleeve had a diameter of 20 mm, and the ratio of the peripheral speed of the developing sleeve to that of the photosensitive drum is set at 1.2:1. With a lower toner circularity, the toner adheres more firmly to the developing sleeve and is more unlikely to fly therefrom when electric fields are applied thereto, and the process speed also increases. An appropriate density (reflection density: 1.35 or more and preferably 1.40 or more) can be maintained only at a process speed of 150 mm/sec. or less, and the device construction must be adapted to a higher process speed.
The reason why the developing capability is degraded as the process speed increases is a decrease in the time required for the developer to pass through the developing section. Thus, efforts have been made to increase the diameter of the developing sleeve or the peripheral speed of the developing speed with respect to the photosensitive drum. However, it should be appreciated that an increase in the size of the device leads to an increase in the size of the image forming apparatus main body. Further, an increase in the peripheral speed of the developing speed with respect to the photosensitive drum results in a decrease in the lifetime of the developing sleeve or an increase in mechanical loads on the toner, thereby degrading the developing capability.
The object of the present invention is to provide an image forming apparatus and a process cartridge using toner that ensures a sufficient developing capability without reducing the lifetime of a developing sleeve even when the process speed (the peripheral speed of a photosensitive member) is increased.
The present invention provides an image forming apparatus comprising an electrophotographic photosensitive member, a charging means for applying voltage to a charge member to charge the electrophotographic photosensitive member, a static latent image forming means for forming a static latent image on the charged electrophotographic photosensitive member, and a developing means for developing the electrostatic latent image,
wherein the developing means is provided with at least a developer holding member for holding a developer containing a toner on its surface and a developer regulating member for regulating a layer thickness of a developer layer on the developer holding member,
the electrophotographic photosensitive member and the developer holding member are set opposite to each other to form a developing section, the developer regulating member regulates the developer to form a thin layer of the developer on the developer holding member surface, and in the developing section, the toner in the developer is transferred to the electrostatic latent image held on the surface of the electrophotographic photosensitive member to form a toner image,
the peripheral speed of the electrophotographic photosensitive member is 150 mm/second or more,
the toner has a weight-average particle diameter of from 5 to 12 xcexcm, and of the toner having a circle-equivalent diameter of 3 xcexcm or more, particles with a circularity a of 0.900 or more found according to formula (1)
circularity a=L0/Lxe2x80x83xe2x80x83(1)
(wherein L0 denotes the circumference of a circle having the same projected area as a particle image, and L denotes the circumference of the particle image) are present at a rate of 90% or more in a number-based cumulative value, and the toner satisfies the following conditions i) or ii):
i) the relationship between the cut rate Z and the weight-average particle diameter X of the toner satisfies expression (2)
cut rate Zxe2x89xa65.3xc3x97Xxe2x80x83xe2x80x83(2)
(wherein the cut rate Z is represented by expression (3)
Z=(1xe2x88x92B/A)xc3x97100xe2x80x83xe2x80x83(3)
where A represents the concentration (the number of particles/xcexcl) of all particles measured with a flow-type particle image analyzer FPIA-1000 manufactured by TOA MEDICAL ELECTRONICS CO.,LTD., and B represents the concentration (the number of particles/xcexcl) of the measured particles the circle-equivalent diameters of which are 3 xcexcm or more), and
the relationship between the number-based cumulative value Y of particles having a circularity of 0.950 or more and the weight-average particle diameter X of the toner satisfies expression (4)
Yxe2x89xa7exp 5.51xc3x97Xxe2x88x920.645xe2x80x83xe2x80x83(4)
(where X is in the range from 5.0 to 12.0 xcexcm); and
ii) the relationship between a cut rate Z and the weight-average particle diameter satisfies expression
cut rate Z greater than 5.3xc3x97Xxe2x80x83xe2x80x83(5)
and the relationship between the number-based cumulative value Y of particles having a circularity of 0.950 or more and a weight-average particle diameter X satisfies expression (6)
Yxe2x89xa7exp 5.37xc3x97Xxe2x88x920.545xe2x80x83xe2x80x83(6)
(where X is in the range from 5.0 to 12.0 xcexcm).
The present invention also provides a process cartridge comprising an electrophotographic photosensitive member, a charging means for applying voltage to a charge member to charge the electrophotographic photosensitive member, and a developing means for developing an electrostatic latent image,
wherein the process cartridge is used for an image forming apparatus in which a toner in a developer is transferred to an static latent image to form a toner image, and the toner image is transferred to a transfer material to form an image, and is so constructed as to be detachably mountable on the apparatus,
the developing means is provided with at least a developer holding member for holding a developer containing a toner on its surface and a developer regulating member for regulating a layer thickness of a developer layer on the developer holding member,
the electrophotographic photosensitive member and the developer holding member are set opposite to each other to form a developing section, the developer regulating member regulates the developer to form a thin layer of the developer on the developer-holding-member surface, and in the developing section the toner in the developer is transferred to the electrostatic latent image held on the surface of the electrophotographic photosensitive member to form a toner image,
the peripheral speed of the electrophotographic photosensitive member is 150 mm/second or more,
the toner has a weight-average particle diameter of from 5 to 12 xcexcm, and of the toner having a circle-equivalent diameter of 3 xcexcm or more, particles with a circularity a of 0.900 or more found according to formula (1)
circularity a=L0/Lxe2x80x83xe2x80x83(1)
(wherein L0 denotes the circumference of a circle having the same projected area as a particle image, and L denotes the circumference of the particle image) are present at a rate of 90% or more in a number-based cumulative value, and the toner satisfies the above conditions i) or ii).